Update preset background example name (#1819)

Author: esimkowitz

docs/docs/presets.mdx

@@ -18,7 +18,7 @@ You can store presets in two locations:
 - `~/.config/waveterm/presets.json`: Main presets file
 - `~/.config/waveterm/presets/`: Directory for organizing presets into separate files
 
-All presets are aggregated regardless of which file they're in, so you can use the `presets` directory to organize them (e.g., `presets/backgrounds.json`, `presets/ai.json`).
+All presets are aggregated regardless of which file they're in, so you can use the `presets` directory to organize them (e.g., `presets/bg.json`, `presets/ai.json`).
 
 :::info
 You can easily edit your presets using the built-in editor: